Corrosion problems in petroleum refining operations associated with naphthenic acid constituents in crude oils have been recognized for many years. Such corrosion is particularly severe in atmospheric and vacuum distillation units at temperatures between 350.degree. F. and 790.degree. F. Other factors that contribute to the corrosivity of crudes containing naphthenic acids include the amount of naphthenic acid present, the concentration of sulfur compounds, the velocity and turbulence of the flow stream in the units, and the location in the unit (e.g., liquid/vapor interface).
In the distillation refining of crude oils, the crude oil is passed successively through a furnace and one or more fractionators such as an atmospheric tower and a vacuum tower. In most operations, naphthenic acid corrosion is not a problem at temperatures below about 350.degree. F. Traditional nitrogen-based filming corrosion inhibitors are not effective at temperatures above 350.degree. F. and the other approaches for preventing naphthenic acid corrosion such as neutralization present operational problems or are not effective.
It should be observed that the term "naphthenic acid" includes mono- and di-basic carboxylic acids and generally constitutes about 50% by weight of the total acidic components in crude oil. Many of the naphthenic acids may be represented by the following formula: ##STR1## where R is an alkyl or cycloalkyl group and n ranges generally from 2 to 10.
Many variations of this structure and molecular weight are possible. Some practitioners include alkyl organic acids within the class of naphthenic acids.
Naphthenic acids are corrosive between the range of about 350.degree. F. (180.degree. C.) to 790.degree. F. (420.degree. C.). At the higher temperatures the naphthenic acids are in the vapor phase and the rate of decarboxylation increases; at the lower temperatures the corrosion rate is not serious. The corrosivity of crude oils and distillates is also affected by the presence of sulfide compounds, such as hydrogen sulfide, mercaptans, elemental sulfur, sulfides, disulfides, polysulfides and thiophenols. Corrosion due to sulfur compounds becomes significant at temperatures as low as 450.degree. F. The catalytic generation of hydrogen sulfide by thermal decomposition of mercaptans has been identified as a cause of sulfidic corrosion.
Efforts to minimize or prevent the naphthenic acid corrosion have included the following approaches:
(a) blending of higher naphthenic acid content oil with oil low in naphthenic acids; PA1 (b) neutralization and removal of naphthenic acids from the oil; and PA1 (c) use of corrosion inhibitors.
Because these approaches have not been entirely satisfactory, the accepted approach in the industry is to construct the distillation unit, or the portions exposed to naphthenic acid corrosion, with the resistant metals such as high quality stainless steel or alloys containing higher amounts of chromium and molybdenum. However, in units not so constructed there is a need to provide inhibition treatment against this type of corrosion. The prior art corrosion inhibitors for naphthenic acid environments include nitrogen-based filming corrosion inhibitors. However, these corrosion inhibitors are relatively ineffective in the high temperature environment of naphthenic acid oils.
Atmospheric and vacuum distillation systems are subject to naphthenic acid corrosion when processing certain crude oils. Currently used treatments are thermally reactive at use temperatures. In the case of phosphorus-based inhibitors, this is thought to lead to a metal phosphate surface film. The film is more resistant to naphthenic acid corrosion than the base steel. These inhibitors are relatively volatile and exhibit fairly narrow distillation ranges. They are fed into a column above or below the point of corrosion depending on the temperature range. Polysulfide inhibitors decompose into complex mixtures of higher and lower polysulfides and, perhaps, elemental sulfur and mercaptans. Thus, the volatility and protection offered is not predictable.